Trying to create a Vault smart-contract in Tron Mainnet that interacts with USDT

Question

I'm trying to create a vault smart-contract that allows deposits and withdraws of Tron USDT. Deposits - work.

But when I try to withdraw, I get ERC20 transfer amount exceedes balance error..... but I gave allowance! like this:

spender = msg.sender; usdt.approve(spender, amount);

And it confirms on blockchain that I gave allowance for SC to spend funds on user behalf. Then I try:

usdt.transferFrom(address(this), recipient, amount);

And it fails! And

usdt.transfer(recipient, amount)

fails too!